Tendon disorders are the most common of all overuse injuries, and injury incidence is related to age. Tendinopathy, for example, is a chronic degenerative condition which is common in the Achilles, patellar and quadriceps tendons, among others. Early detection of tendon injuries is extremely challenging, and end-stage treatments have limited success, often leading to a downward spiral of disuse atrophy. Although the effects of aging on muscle morphology, strength and composition are well documented, the age-related changes that occur in tendon are poorly understood. Recently, ultrasound has gained use as a technique to evaluate the mechanical changes in tendon with aging, but have produced conflicting results. The lack of conclusive evidence is likely due in part to the limited capacity of current techniques to accurately capture in vivo tendon mechanics. Specifically, current methods are incapable of measuring regional tendon strains, which have been suggested to contribute to tendon injuries. The purpose of this study, therefore, is to advance the technique of quantitative 2D ultrasound elastography for the evaluation of regional tendon strains. The muscle-tendon- specific elastography technique will be validated with a novel tendon-mimicking phantom, finite element model and ex vivo testing. Following technique validation, the age-related changes in tendon regional strain will be evaluated by collecting ultrasound data during dynamic loading of the healthy Achilles tendon in three age groups: young, middle-aged, and older adults. The novel insights gained from this study will advance the scientific understanding of age-related tendon disorders, and will introduce an innovative technique for tracking strain nonuniformities which are relevant to understanding, diagnosing and treating tendon injuries. PUBLIC HEALTH RELEVANCE: This purpose of this study is to validate and use a new quantitative ultrasound technique to investigate how aging alters tendon properties and deformation patterns. The study will provide insight into the underlying causes of tendinopathy, a chronic painful injury that is more common among middle-aged and older adults. Furthermore, the ultrasound techniques could be used to improve the early diagnosis of tendon disorders, and monitor the effects of clinical treatment on tendon function.